1. Field of the Invention
Embodiments disclosed herein relate generally to methods cleaning wellbores. In particular, embodiments disclosed herein relate to cleaning fluids capable of cleaning wellbores.
2. Background Art
During the drilling of a wellbore, various fluids are typically used in the well for a variety of functions. The fluids may be circulated through a drill pipe and drill bit into the wellbore, and then may subsequently flow upward through wellbore to the surface. During this circulation, the drilling fluid may act to remove drill cuttings from the bottom of the hole to the surface, to suspend cuttings and weighting material when circulation is interrupted, to control subsurface pressures, to maintain the integrity of the wellbore until the well section is cased and cemented, to isolate the fluids from the formation by providing sufficient hydrostatic pressure to prevent the ingress of formation fluids into the wellbore, to cool and lubricate the drill string and bit, and/or to maximize penetration rate.
Often drilling of a well is begun with a water-based fluid. However, as drilling depth increases, formation pressures and heat increase, and these conditions necessitate use of a second fluid, often an oil-based fluid. During the replacement (displacement) of one fluid for another, spacer fluids and/or displacement fluids are used to provide a physical barrier between the old and new fluid, and also service to clean the well of residuals of the old fluid. In open hole wells, upon drilling to a total depth, the oil-based fluid must be removed from the wellbore, and the wellbore prepared for production, namely by cleaning the well of residual fluid, removing filtercake, and installing completion equipment.
In cased holes, upon drilling a predetermined length successfully, the wellbore may typically prepared for its completions phase by isolating formations transversed by the wellbore with a casing string or liner. Specifically, the bit and drill string are removed from the well, and a larger diameter string of casing or liner is inserted and secured therein with cement. The casing also insures permanence of the borehole and prevents entry of formation fluid into the wellbore, other than from the formation that is being produced. Similar to open hole wells, the casing must be cleansed to remove residual fluid from the walls.
Before the well can be completed and put into production, it is necessary to clean the wellbore and replace the fluids present in the wellbore with a completion fluid such as brine. The cleaning process serves to remove residual fluid build-up on the walls of the wellbore (including the casing or liner), as well as solids present in the well. Furthermore cleaning of the casing wall may also be necessary at intervals during well production to remove accumulated debris and residues, such as oil, paraffin and scale. Debris and residues on the internal walls of the well may have a negative impact on well productivity.
Cleaning of the wellbore typically occurs through the use of spacer fluids and cleaning washes. A spacer is generally characterized as a thickened composition that functions primarily as a fluid piston in displacing fluids present in the wellbore. Frequently, spacers contain an appreciable amount of weighting materials, as well as fluid loss control agents. Cleaning washes or fluids, on the other hand, are generally thin, or relatively non-viscous, fluids that are effective principally as a result of turbulence, dilution, and surfactant action on the residual fluid and filter cake. Cleaning fluids may contain some solids to act as an abrasive, but the solids content is generally significantly lower than in spacers because cleaning fluids are typically too low in viscosity to have good solids carrying capacity. Such cleaning fluids are particularly necessary for the transition between an oil-based wellbore fluid to a water-based wellbore fluid, so that substantially all oily residue on borehole surfaces may be removed.
Accordingly, there exists a continuing need for development of cleaning fluids that can be used in the displacement of wellbore fluids and leaving the borehole surfaces clean.